It is likely that other examples of isodiagonality remain to be explored. In fact, isodiagonality seems to occur in many parts of the Periodic Table. For example, there are similarities between the chemistry of tellurium(VI) and astatine(VII) (Figure 11.7).
These resemblances are not just in formulas, but also in the pH dependence of the oxo-anions (Table 11.5).
Evidence-Based Isodiagonality
It is probable that at least one similarity could be found between any pair of elements. Thus, it is important to define criteria that can be used to establish an isodiagonal relationship as a real phenomenon. There seems to be three separate ways that can be used individually — or preferably in combination — to define elements as having an isodiagonal relationship.
•That for several compounds, there are diagonal similarities in physical and/or chemical properties. This is the classic definition used for the Li–Mg, Be–Al, and B–Si relationships.
•That very unusual chemical structures are shared in a diagonal pattern. Examples of these would be the tricarbide(4−) found for the Li–Mg pair and the cis-dioxocations of the V–Mo pair.
•That in a few cases, there is an isodiagonal relationship between two elements that share the same biological function or occupy the same enzyme site. Examples of this would be the Li–Mg pair and the V–Mo pair, both discussed earlier.
Evidence of isodiagonality is usually found by considering valence-isoelectronic species, but for elements that have more than one oxidation state, similarities can sometimes be found for a common oxidation state.
Commentary
It would be nice to discover an all-encompassing explanation for isodiagonal relationships. As they are an upper-left–lower-right pattern, electronegativity might be used to account for some of the resemblances, while similarities in ionic radius and/or charge density might explain others. However, there seems to be no simple general “explanation” for isodiagonality. From the perspective of this series, however, the more important task is to define and clarify the isodiagonal relationship as one of the valid linkages among the chemical elements — and not just limited to the upper-left corner of the Periodic Table.
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